1. Field of the Invention
The present invention is generally in the field of semiconductor fabrication. More specifically, the present invention is in the field of semiconductor wafer fabrication.
2. Background Art
On-resistance can be an important operating parameter for semiconductor devices. The on-resistance of a power metal-oxide-semiconductor field-effect transistor (MOSFET), for example, is typically recognized to include resistance contributions from the semiconductor wafer substrate, the epitaxial semiconductor channel, and contributions from packaging. One approach to improving on-resistance includes reducing the wafer substrate resistance contribution by reducing the thickness of the wafer. However, care must be taken when thinning the wafer substrate to avoid variation in substrate thickness across the wafer, because a substantially uniform wafer thickness facilitates a desirably uniform on-resistance distribution for the power MOSFET devices fabricated on the wafer.
The manufacturing process that typically defines wafer thickness is backgrind. A backgrind process utilizes abrasives on grinding wheels to remove semiconductor substrate material from the back surface of a wafer and thereby reduce its initial thickness to a desirable thickness for improving device on-resistance. The backgrind processes typically in use are designed to minimize substrate thickness variation for wafers having a smooth front surface. Normally, wafer front surfaces are laminated with a polymeric tape (known as backgrind tape) that both protects the front surface of the wafer from debris on the grinding chuck and also has the capability to absorb a limited amount of surface irregularity on the front surface of the wafer, so that those irregularities are not transferred to the back surface during backgrind.
However, in some fabrication processes it may be desirable to perform the backgrind operation on wafers after solder bumps have been formed on the front surface. The solder bumps are typically disposed on device pads across the wafer front surface. The application of backgrind tape under these conditions may absorb the topography produced by the solder bumps to some extent, but transfer of irregularities remaining on the front surface often still occurs due to the relatively large size of the solder bumps, resulting in undesirable variation in substrate thickness after backgrind.
Thus, there is a need to overcome the drawbacks and deficiencies in the art by enabling a semiconductor wafer with solder bumps formed thereon to undergo a backgrind or other thinning process without suffering substantial substrate thickness variation as a result.